The structure of oxytocin in one crystalline form will be determined. A non-twined crystalline salt of the 4' -hydroxy 5' -chloro-4 azobenzene sulfonic acid has been prepared and shown to be isomorphous with the crystalline 5' -bromo analog. The structural analysis initiated by single isomorphous replacement techniques will proceed by iterative Fourier and tangent refinement methods. The question of oxytocin molecular flexibility will be investigated in the solid state by structure analyses of a crystalline non-isomorphous oxytocin salt, and of crystalline deamino-oxytocin. Rigid-body vector search techniques and direct methods will be employed. There are many oxytocin and vasopressin analogs and we shall attempt to obtain crystalline preparations of some of these to study conformational stability. The structure of the isolated ring will also be studied. If crystalline preparations of the neurophysin-oxytocin complex can be obtained a structure study will be begun.